The demand for propylene increasingly exceeds the supply available from petroleum refining, so a number of on-purpose propylene production processes have been developed. One important process converts propane to propylene by catalytic dehydrogenation. Under suitable conditions of low pressure and elevated temperature, dehydrogenation proceeds according to the reaction:C3H8=C3H6+H2.
Most propylene is subsequently converted to polypropylene, a thermoplastic polymer used in a wide variety of applications, and for which the global market exceeds 45 million metric tons.
Likewise, the demand for iso-butene (iso-butylene) exceeds the supply available from refinery streams. On-purpose iso-butylene is manufactured from iso-butane, also by catalytic dehydrogenation at low pressure and elevated temperature, according to the reaction:C4H10=C4H8+H2.
Iso-butene is an intermediate in the production of a variety of products. It is reacted with methanol and ethanol in the manufacture of the gasoline oxygenates methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). Polymerization of iso-butene produces butyl rubber (polyisobutylene).
Industrial dehydrogenation processes are run at low pressure to maximize the conversion of paraffin to olefin. Nevertheless, the per pass conversion is often only around 50%. In this particular case, the reaction product will contain 0.5 mols of olefin, 0.5 mols of co-product hydrogen and 0.5 mols of unreacted paraffin. Side reactions also occur, forming small amounts of lighter hydrocarbons such as ethane and methane. In addition, because the reactions are typically run at sub-atmospheric pressure, ingress of small quantities of air can contaminate the reaction products with nitrogen and oxygen, and some of the oxygen may react to produce water and carbon oxides.
The desired reaction product is commonly recovered from the mixture of product gases by cooling, followed by compression to a pressure at which the least volatile components—the product olefin and the un-reacted feed paraffin—can be condensed out by further cooling. The principal drawback to this process is that, as the olefins and paraffins condense, their partial pressure in the gas stream decreases, lowering the hydrocarbon dewpoint of the gas stream. To achieve high levels of hydrocarbon recovery, therefore, requires deep cooling of the gas stream to temperatures far below the initial dewpoint of the reaction mixture. Temperatures down to as low as −40° C. or below are routinely employed, necessitating costly pretreatment of the gas (to avoid freezing of water vapor and carbon dioxide), costly materials of construction (to avoid embrittlement problems), and expensive and complex ways to provide refrigeration, such as the use of cryogenic turboexpanders.
After very low temperature or cryogenic condensation, the recovered liquid is stabilized by stripping out lighter components and the resultant olefin/paraffin mixture is distilled to separate the olefin product from the unreacted paraffin feed, which is recycled to the reaction step. The uncondensed gases, predominantly hydrogen, are used as fuel or may be further purified, for example by pressure swing adsorption.
Various designs incorporating separation membranes have been proposed for improving dehydrogenation processes.
U.S. Pat. No. 7,405,338, to Brophy et al. (Velocys), discloses a method of dehydrogenating hydrocarbons to yield unsaturated compounds. The method reduces coking in the catalyst bed and allows for stable, relatively long-term operation in small reactors. In some embodiments, a hydrogen-permeable membrane is used to selectively remove hydrogen.
U.S. Pat. Nos. 5,516,961 and 5,430,218, to Miller et al. (Chevron), disclose processes for catalyst dehydrogenation of light paraffinic hydrocarbons using a catalyst comprising a noble metal and an intermediate pore size zeolite having a specified alkali content. The processes may include a membrane separation step for separating hydrogen from the effluent of the dehydrogenation reaction. The polymer-porous solid composite membrane may be, e.g., a porous ceramic material coated with a fluorinated dianhydride-diamine, a fluorinated polycarbonate or fluorinated polysulfone.
U.S. PG Pub. Nos. 2012/0190904 and 2012/0078204, to Butler et al. (Fina Technology, Inc.), disclose dehydrogenation methods that include passing a dehydrogenation product through a membrane separator and permeating hydrogen through a membrane positioned in the membrane separator. In certain embodiments, the membrane is inorganic and formed of either ceramic or a sintered metal.
Chinese Patent Application CN102795956(A), to Wilson Engineering, discloses a method for separating reaction products produced during preparation of propylene by dehydrogenating propane. In the method, reaction products produced during preparation of propylene by dehydrogenating propane are separated by combining membrane separation with cryogenic separation.
European Patent Application EP2505573A1, to Stamicarbon B. V., discloses a process for the catalytic dehydrogenation of alkanes so as to form the corresponding olefins. The reaction mixture is subjected to membrane separation of hydrogen, in a separate unit. Preferably a plurality of alternating reaction and separation units is used. The process of the invention serves the purpose of reducing coke formation on the catalyst, and also of achieving a higher alkane conversion without a similar increase in coke formation. The process can also be used for the production of hydrogen.
Given the huge demand for C3 and C4 olefins, and the complexity and cost of low-temperature and cryogenic operations, it would be highly desirable to improve the product recovery train to avoid the use of low-temperature separation steps as far as possible. There remains an unmet need for processes that can recover light olefins and paraffins from raw reaction mixtures in a simple, cost and energy efficient manner.